A typical roller cone earth boring bit, such as used to drill wells, has three cones that roll around a common axis. The cones are mounted to bearing pins that depend from head sections. The head sections are welded together to form a body that is threaded at the upper end for connection to a drill string.
FIGS. 1-3 illustrate a typical prior art rolling cone bit 11. Bit 11 has three cones 13, 15 and 17. Cone 13 has a spear point cutting element 19 on its inner end and a heel or outer row 21 of cutting elements on its outer end. The outer side of each tooth of outer row 21 joins a gage surface 22. The cutting elements in this instance comprise teeth that are integrally formed with cone 13 and milled into desired shapes. Milled teeth are generally chisel-shaped, each having a crest 28 that is perpendicular to the direction of rotation of the bit. Alternately, the cutting elements could be cast with the body of the cone or comprise tungsten carbide inserts pressed into mating holes.
Cone 13 also has an inner row 23 spaced a short distance from outer row 21. A groove 25 locates between outer row 21 and inner row 23. A layer of hardfacing 27, shown by phantom lines, covers each cutting element in outer row 21 and inner row 23. Groove 25 is generally triangular in cross-section and has a width 26 that may be measured between tips of teeth 21, 23 at the crests 28. In the prior art, width 26 is normally less than the width of crest 28 of a cutting element of inner row 23 or of outer row 21.
Referring to FIG. 1, cone 15 has an outer row 29 and an inner row 31 spaced apart by a groove 33. Groove 33 has a much wider width 34 than width 26 of cone 13. In the prior art, width 34 is typically equal or greater than the width of crest 28 of one of the teeth of inner row 31. Cone 17 has an outer row 35 and an inner row 37 spaced apart by a groove 39. Groove 39 has a width 40 that is wider than width 34 and width 26. Width 40 is greater than the width of crest 28 of one of the outer row teeth 35 or inner row teeth 37.
The various rows 21, 23, 29, 31, 35 and 37 are arranged for a desired bottom hole coverage, as indicated in FIG. 2. In FIG. 2, all of the rows of teeth are rotated into a single sectional plane. Some of the teeth intermesh with each other as shown in FIG. 1. The number of rows per cone in the prior art can be more or less than those shown in FIG. 1. In the prior art example shown, there are a total of seven rows, and the narrowest groove width 26 is located on cone number one, which has the spear point. In an eight row bit, the narrowest groove width 26 would be normally on cone 17, which is cone number two. In a nine row bit, the narrowest groove width 26 would be on cone 15, which is cone number three. A narrow groove on one of the cones has been necessary in the prior art in order to achieve intermesh and the desired bottom coverage. While workable, in certain formations such as shales, the cuttings tend to ball up in rows separated by narrow grooves, reducing the rate of penetration.